Sealant product, laminate thereof, and pneumatic tire constructed therewith

ABSTRACT

A carcass for a self-sealing pneumatic tire is constructed by starting with an extruded thin flat strip of puncture sealant both surfaces of which are detackified. The strip is place on a building drum and its upper surface is wiped clean so that an inner liner may be adhered to it. Thereafter, the carcass is conventionally completed with superimposed successive layers of belts, and tread along with bead rings, etc. The carcass is then cured in a conventional curing press so that the sealant is cured by contact with the curing bladder. The curing sealant is not restrained in any way, but does not flow. Alternatively, a laminate of the sealant is formed with the inner liner, with only the exposed surface of the sealant detackified so that it adheres neither to the building drum nor to the curing bladder. The detackifier remains on the sealant. The recipe for this effective sealant product requires a ratio of high molecular weight elastomer to low molecular weight elastomer greater than 1; also, that there be at least as much, by weight, processing aid as there is high mol wt elastomer. Such a recipe has a room temperature peak Mooney viscosity above 70°, and, after curing, a peak Mooney viscosity at 150° F. in the range above 15° but below 30°. A combination of homogenizer and tackifier facilitates maintaining the necessary viscosity of the uncured sealant recipe. The tire provides excellent self-sealing results.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of Ser. No.044,628 filed May 1, 1987 which is in turn a divisional of Ser. No.693,090 filed Jan. 21, 1985 now issued on May 12, 1987 as U.S. Pat. No.4,664,168. This is a division of application Ser. No. 080,842, filedAug. 3, 1987.

BACKGROUND OF THE INVENTION

This invention relates to an improvement in self-sealing tubelesspneumatic tires constructed upon a laminate of elastomers comprising anair-impervious liner or elastomeric strip ("inner liner") coated with athin layer of bare (that is, not covered and not constrained by a sheetof flexible material), elastomeric puncture-sealing material("sealant"). The laminate of elastomers, namely the inner liner coatedwith the sealant is referred to herein as the elastomer laminate("laminate" for brevity) in which the upper surface of the sealant isdetackified with a detackifier which prevents adhesion of the surface tometal, and also to a hot, pressurized curing bladder. The uncured(uncrosslinked) sealant is essentially free of crosslinks. The laminatecan be used as the first structural component of a tire to beconstructed on a building drum because the detackified upper surface ofthe sealant does not stick to the drum.

In those instances where sealant is desired without the inner liner,this invention relates to a relatively thin generally laminar extrudate,less than about 10 mm thick, and at least 10 times as wide as it isthick, of an extruded sealant product which has unique flow (viscosity)properties. However, as extruded, the sealant strip has excellentadhesive qualities which must be countered by coating its upper andlower surfaces with detackifier. Thus surface-detackified, a desiredlength of the sealant product strip may be cut and placed on a buildingdrum to initiate the building of a tire carcass. After the sealantproduct strip is placed upon the drum, the detackifier is removed fromthe upper surface of the strip, and then, an inner liner isconventionally wrapped over the strip, followed thereafter by successivelayers which form the carcass.

This invention derives from the desire to eliminate the flow-restrainingmeans referred to as "edge strips" in our copending patent applicationSer. No. 044,628,090 filed May 1, 1987, and U.S. Pat. No. 4,664,168. Theedge strips were necessary because we did not know how to formulate thesealant so that it would not flow under heat and pressure in the curingpress. We were unaware of the critical importance of maintaining theweight ratio of high molecular weight (mol wt) elastomer to low mol wtelastomer greater than 1, nor were we aware of the role of he"tackifier" (so termed in U.S. Pat. Nos. 3,981,342; 4,064,922; and4,115,172), having regarded the tackifier simply as a `processing aid`with little, if any, effect on viscosity.

Particularly, we used a mixture of tackifiers, including the samemixture of Struktol® 30 and Piccopale® resin as used in the '922 patent,to provide the particular balance of uncured and cured physicalproperties taught therein, recognizing that neither tackifiercontributed to the cured Mooney viscosity of the sealant, but eachgreatly affected the properties of the sealant. Attempts to raise theviscosity of the uncured '922 sealant so that it would essentially notflow under curing conditions, yet to lower its cured viscosity so thatit would seal punctures more effectively, prompted an investigation intonumerous modifiers and processing aids. During this investigation, wefound that, when we increased the ratio of high mol wt elastomer to lowmol wt elastomer in the mixture, the viscosity increased, both beforeand after curing the mixture, which is what we expected. This increasein viscosity, obtained by increasing the relative amount of high mol wtelastomer, was too high to give satisfactory self-sealing of punctures.It was therefore particularly surprising that, as long as there was morehigh mol wt. than low mol wt. elastomer, by using more processing aid inthe sealant recipe than used in the '922 patent, whether the processingaid was a tackifier or homogenizer, the viscosity of the uncured sealantwas increased, and that of the cured sealant was decreased so as to giveexcellent self-sealing.

Further, though acceptable results are obtained with several differenttackifiers, and with several combinations of different tackifiers, theexcellent results obtained with a homogenizer in combination with atackifier was overlooked. In particular, the homogenizer's peculiar rolerelative to the amount of tackifier, in influencing viscosity under heatand pressure was not appreciated.

A tackifier is a material which increases tack but has little effect onend properties after cure. A homogenizer is a material which improvesthe blending characteristics of elastomers of dissimilar polarity orviscosity, also with little effect on end properties after cure. Becausetackifiers and homogenizers are low mol wt compounds it was to beexpected that they would have no effect on the cured properties of thesealant mixture (recipe). Consistent with their combined presumed`non-effect` on cured properties, the teaching of the '922 patentregarded its combination "tackifier" as being a diluent for the low molwt elastomer, requiring that the sum of the weights of low mol wtelastomer and tackifier be greater than that of the high mol wtelastomer.

It is therefore particularly surprising that, as long as there is amajor proportion of high mol wt elastomer present, relative to the lowmol wt material, it is not critical which tackifier or homogenizer, orwhat combination of tackifiers, or of homogenizers, or which combinationof homogenizer and tackifier is used, provided the peak Mooney viscosityof the recipe is maintained in the range specified. This peak Mooneyviscosity at 75° F. (room temperature) ("MLP/rt") is in the range fromabove 70 to about 110, and is best maintained with a weight ratio ofhomogenizer to tackifier in a specified range. In this range, thepronounced effect on viscosity, both before and after curing the sealantmixture, is such that the edge strips of our '168 patent can beeliminated.

As was disclosed in the '922 patent to Farber et al., a mixture of highand low mol weight elastomers, the latter being present in an amountmore than 50% by weight based on the combined weight of the high and lowmol wt elastomers, and tackifier or plasticizer, and, cured to a limitedextent, could adequately control flow under conditions of use of(running) the tire. The high mol wt elastomer furnished rigidity andstrength, and the low mol wt furnished adhesion and conformability. Byincreasing the proportion of high mol wt elastomer, the tendency of flowattributable to the low mol wt elastomer is decreased but not completelyremoved. Therefore, the '922 mixture was partially cured, allowing thehigh mol wt elastomer to act as supporting structure to retard flow,without crosslinking the low mol wt elastomer to the point where itsability to function as sealant would be significantly impaired.

The function of the low mol wt elastomer as sealant was misdirected. Wehave found that, as is illustrated in an extreme case to establish thefact, there may be only one-tenth as much low mol wt elastomer as high,provided there is an appropriate choice of tackifier(s); in the bestmode, there is more homogenizer than tackifier present.

Moreover, the flow properties of uncured sealant as now formulated,permits it to be cured in a green carcass with minimal flow onto thesidewalls of the carcass as it is vulcanized ("cures") in contact with ahot pressurized bladder in a curing press, only because the detackifyingfunction of the detackifier is not vitiated by the heat and pressure.Because the uncured '922 sealant, as an integral part of a greencarcass, could not be cured in a curing press, the less preferredembodiment of the '922 invention required that a flat strip of extrudedsealant be first cured, then incorporated on top of the liner in anuncured steel belted radial tire which was cured in a conventional tirepress. In example VII of the '922 patent, the strip was cured byirradiation with a 1.4 million volt electron beam at a dosage of 20megarads. The cured strip was placed on top of the liner inside a greentire, and the tire could be cured in a conventional tire press becausethe bladder did not adhere to the cured sealant on the liner. But curingthe sealant on the inner liner also cures the inner liner withpredictable results in a tire using the cured laminate.

It became imperative that the laminate not be precured for severalreasons. To begin with, precuring the laminate resulted in using aprecured inner liner to build a tire. One skilled in the art willappreciate that when a tire is expanded in the second stage, a precuredinner liner will not have the elasticity required of it. But no attemptwas made to cure an uncured laminate partly because it was evident thatthe sealant flowed uncontrollably during cure, and partly because nodetackifier was found which readily provided the several featuresdemanded of it. A detackifier was required to prevent adhesion of thesealant surface to a metal building drum, and also to prevent adhesionof the curing sealant (at about 350° F.) to the hot pressurized curingbladder under typical operations in a conventional curing cycle. It isevident that if the sealant adhered to the drum, whether it adhered tothe curing bladder was immaterial. In addition, the detackifier was toremain on the surface of the cured sealant, rather than beingtransferred to the curing bladder, because a non-detackified sealantsurface is deemed more desirable to seal a puncture caused by a nailwhich, after piercing the tire's tread and sealant, is ejected fromtire.

Hence, in the more preferred embodiment of the '922 invention, sealant(referred to as "post-cure sealant" because sealant was deposited in acured tire) was extruded onto the inner liner of a cured tire, asdescribed in detail in U.S. Pat. No. 4,115,172 to Baboff et al., so thatthe sealant adheres to the inner circumferential surface of the crown,on the opposite side of the road-contacting tread surface, and thesealant was cured by heating for several days at a temperature highenough to cure the sealant, typically about 150° F., because the curingagent (curative) used was a tetraalkyl titanate

Later attempts to cure an uncured laminate were made after the discoveryof a commercially available material was found to be an effectivedetackifier. Still, all attempts to cure the laminate in the curingpress resulted in unacceptable, if not uncontrolled flow of the curingsealant in the tire being cured. We addressed the flow problem duringcure in our '168 patent by the use of flow-restraining edge strips.

The equivocal results obtained, coupled with the adverse economics ofsuch post-cure sealant deposition, particularly as taught in the '342and '922 patents to Farber et al., was followed by depositing thesealant before molding and curing a green tire carcass, referred to aspre-cure sealant; but the sealant was crosslinked to increase itsviscosity, then was covered with a layer of flexible material to preventadhesion of the sealant to the drum, as disclosed in the '342 and '922patents. Covering the sealant caused blistering during cure. It becamenecessary in the prior art patents, to adhere the sealant and flexiblematerial to the tire in a separate operation, after the tire was moldedand cured.

Eventually, the economics of the additional operations for a crosslinkedsealant being deposited after curing, led to a partially uncoveredsealant constrained by edge strips. The edge strips restrain flow of thesealant during contact with a curing bladder under curing conditionsnamely about 350° F. and about 350 psig. This latter concept, utilizingthe '922 sealant composition, is disclosed in our '168 patent.

In the best mode, we have added the homogenizer to the ingredients ofthe '922 sealant because we found that a recipe in which the sum of theweights of homogenizer and tackifier is greater than that of any othersingle component of the recipe, increases the viscosity of the sealantso as to essentially negate its flow during curing of the tire, yetpermits the cured sealant (after the tire is cured) to flow enough toseal a puncture during use of the tire mounted on an operating vehicle.

The sealant laminated to an inner liner, disclosed in the '922 patent,comprised a blend of a major proportion of a low mol wt liquid elastomermixed with a tackifier or plasticizer, and a minor amount by weight of ahigh mol wt elastomer. The blend, when crosslinked, had a peak Mooneyviscosity of from 30 to 55 ML at 150° F. (MLP/150). By trial and error,Farber et al had determined that a cross-linked sealant composition witha peak Mooney viscosity outside the range specified, would be unusableas a sealant. If the viscosity was lower than 30 MLP/150, the sealantwould tend to flow down from the shoulder and sidewall areas of the tirewhen it is run at high speed as well as out of the hole when the tire ispunctured. If the viscosity was higher, it would not flow sufficientlyto seal a puncture.

Even at the high end of the range, the sealant was too fluid underconventional curing-press conditions. We coped with the problem, asdisclosed in our '168 patent by providing sealant-retaining edge strips,carried by the inner surfaces of the sidewalls of the tire. These stripsfunctioned as flow-restraining means because the viscosity of the '922sealant was low enough to flow during curing of the tire, and requiredto be restrained.

Notwithstanding the effectiveness of the edge strips the difficulty ofreliably and reproducibly manufacturing tires with such edge stripsresulted in unfavorable economics of production. The decision was madeto eliminate the edge strips in favor of renewing the search for acrosslinked sealant composition which does not flow within the greencarcass when it is being cured in a curing press, and the bare surfaceof which crosslinked composition, upon curing, does not stick to thetire-building drum, yet after the tire is cured, the sealant is fluidenough to plug a puncture.

This invention embodies the culmination of that search.

The '342 patent, like the '922 patent, used a major amount of low mol wtelastomer relative to the high mol wt, but unlike the '922 patent, usedno tackifier or plasticizer. The result obtained in the '342 patent was"to give an initial Mooney viscosity at room temperature (the initialpeak reading attained which is usually within the first few seconds) ofbetween 30 and 70 (large rotor, ML) in the final crosslinked mixture,with a preferred range of 40 to 60. Below 30 ML, the composition wouldtend to flow down from the shoulder and sidewall areas of the tire whenit is run at high speed, as well as out of the hole when the tire waspunctured. Above 70 ML, the sealant capability of the composition issufficiently impaired to render it unusable for practical purposes".(see btm of col 3). The viscosities given were incorrectly given asbeing at room temperature when in fact they were measured at 150° F.--acorrection made in the '922 patent discussed hereinbefore.

SUMMARY OF THE INVENTION

It has been discovered that a particular sealant product, substantiallyfree of crosslinks, when laminated to an air-impermeable elastomericstrip used as an inner liner, may in turn be laminated to the first ofplural superimposed belts to form a green tire carcass which may then beconventionally cured in a curing press, without the sealant beingcovered, and without requiring that the liner be equipped with any meansto restrain the flow of sealant, while it is curing ("curing sealant"),when the tire is in the press.

It is therefore a general object of the present invention to provide animproved self-sealing pneumatic tire with a layer of bare sealantmaterial laminated to an inner liner to form an elastomeric laminatefree of sealant restraining means, and the liner may be integrated intothe tire structure prior to molding and curing the tire.

It is also a general object of this invention to provide asurface-detackified, uncrosslinked sealant product consistingessentially of (i) a blend of a major amount by weight of high mol wtelastomer, and a minor amount by weight of low mol wt elastomer, basedon the combined weight of the high and low mol wt elastomers; and, (ii)at least as much processing aid, whether homogenizer or tackifier, asthere is high mol wt elastomer; and, (iii) a curative in an amountsufficient to provide said sealant with a peak room temperature (75° F.)Mooney viscosity (MLP/rt) in the range from above 70 to about 110, and,when cured, with a peak Mooney viscosity at 150° F. (MLP/150) below 30,preferably in the range from above about 15 but less than 30.

It is also a general object of this invention to provide a laminateformed by continuously extruding a generally flat strip of sealant ontoa web to which the lower surface of the sealant adheres. The uppersurface is coated (surface-detackified) with a film of detackifier. Theweb is preferably liner (butyl rubber) stock or carcass (natural rubber)stock, typically less than about 8 mm thick, used to provide anair-impermeable elastomeric liner within the carcass. The laminate isthen cut to a predetermined length to encircle a tire-building drum. Tohandle the laminate conveniently, the web is placed on a co-continuousbacking sheet of synthetic resinous material, preferably a polyolefinsuch as polyethylene having a thickness of from about 1-5 mils, and awidth slightly greater than that of the inner liner (web). The laminateis substituted for a conventional inner liner in the usual constructionof a green carcass which is then cured in a curing press. In the curedtire, the circumferential edges of cured sealant are smoothly blendedinto the sidewalls, but flow of sealant is so limited that the thicknessof the sealant in the region of the circumferential centerline is noless than 80% of the thickness of the uncured sealant just before beingplaced in the curing press.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present inventionwill be more clearly understood from the following detailed descriptionthereof when read in conjunction with the accompanying drawings inwhich:

FIG. 1 is a cross-sectional elevation view of a self-sealing pneumatictire diagrammatically illustrating a layer of sealant on a singleelastomeric inner liner.

FIG. 2 is a cross-sectional elevation view of another embodiment of aself-sealing pneumatic tire diagrammatically illustrating a layer ofsealant on dual elastomeric liners which result in sealant laminated toa laminate of inner liners.

FIG. 3 is a cross-sectional elevation view of a laminate structure inaccordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is diagrammatically illustrated across-sectional view of a tubeless tire 10 constructed in accordancewith the subject invention. The tire 10 includes a tire carcass 11 whichmight be composed of radial or substantially radial superimposed carcassplys of conventional form. The carcass 11 terminates at its innermostedges in a pair of beads 12 and 12'. The lateral regions of the carcassform sidewall regions 14 and 14' which generally form or define themaximum width of the tire 10. A tread region 16 forms a crown region ofthe carcass 11 and a pair of buttress regions 17 and 17' are positionedbetween and connect the tread 16 and sidewalls 14 and 14'. In theinterior of the tread region 16, there is generally found one or morebelts 18 that are provided to enhance the performance and durability ofthe tire 10. The surface region of the tread 16 forms a tread pattern19. The inner surface 13 of the tire is generally covered by anair-impermeable inner liner 24 made of, for example, butyl rubber.

The cord ply components of the tire 10 are usually made of layers ofrubberized cords or cables made of any suitable natural or syntheticfiber such as nylon, rayon, polyester, metal wire, glass fiber, etc.,and each of the plys of the carcass 11 extends from bead 12 to bead 12'.The belts 18 can also be made of any of several materials, includingglass fiber, aromatic polyamide, metal or rayon.

The description of the tire thus far is basically conventional andgenerally in accordance with the current state of the art.

The novel tire 10, illustrated in FIG. 1 is of the self-sealing variety.The mid-equatorial plane of the tire is indicated by a dashed line X--X.A flat strip or layer (to distinguish it from the elastomer strip usedas inner liner) of sealant 20 is adhered to the inner surface of theinner liner 24, covering its inner surface where it is most likely to bepunctured (puncture region), namely between sidewalls 14 and 14'.Typically, the layer is less than 0.5 in thick, and much wider than itis thick, being usually at least ten times wider than it is thick, andit covers from about 20% to substantially the entire area of the liner'supper surface. The elastomeric inner liner 24 and the elastomericsealant 20 together form an elastomeric laminate 22 which is the firstcomponent of the tire as it is built on a building drum.

In the cured tire, boundaries 23 and 23' of the cured sealant aresmoothly blended into the sidewalls due to the high pressure at theraised edges of the substantially uniformly flat sealant, preferablyfrom about 0.1"to about 0.25" thick, the pressure being exerted by thehot bladder used in a curing press. Preferably from about 30% to about50% of the inner surface of the tire may be covered with sealant, itbeing apparent that depositing the sealant where no puncture is likely,serves no useful function.

The upper surface of uncured sealant in the tire carcass is coated witha detackifier 21, present as a thin film in an amount sufficient toprevent adhesion of the surface to a building drum, and after the tireis built, to a hot curing bladder. The detackifier, which isnon-degradable under curing conditions, is not a self-supporting filmbut preferably an organic polymer conveniently applied as a sprayablesolution, and, for evident reasons, is an essential part of the sealantproduct. The composition of the detackifier is not narrowly criticalprovided it is non-reactive with the sealant and is used in an amountsufficient to fulfill its above-stated functions. Some release agentssuch as a silicone spray or zinc stearate are also effectivedetackifiers if applied to the sealant surface. Upon the tire beingcured, the detackifier is maintained on the surface of the sealant.

Referring now to FIG. 2, an alternate embodiment of the subjectinvention is shown in which the inner surface of the tire is formed by acombination of two inner liners; a first inner liner 24, and a secondrelatively narrower inner liner 24' to which the sealant 20 is in turnlaminated to form a laminate 27. The surface of the second inner liner24' is substantially completely covered with sealant except for thenarrow strips 25 and 25' of uncovered liner 24' into which the sealantis smoothly blended after the tire is cured. Using a relatively narrowinner liner facilitates handling the laminate 27 (sealant to inner liner24') and the laminate's accurate placement on a building drum. The firstliner 24 is the conventional "full" inner liner which is then placedover the liner 24' of the laminate 27 during construction of a typicalsingle stage radial steel-belted tire, followed by the belts and tread,as is well known in the art and described in U.S. Pat. No. 3,489,634,inter alia.

In a two stage process for building a tire, the green carcass containingthe laminate is transferred to a shaping mechanism for pre-shaping thecarcass into a toroid and to its approximate final diameter. A belt andtread assembly is then added, and the green carcass is then transferredto a vulcanization press for final shaping and curing in a conventionalmanner.

Referring now to FIG. 3, there is diagrammatically illustrated a crosssection of a laminate 27 comprising the inner liner 24' to which sealant20 is adhesively secured due to its adhesive quality. The sealant isextruded continuously from a slit die in an extruder which extrudes thesealant on to continuous liner or carcass stock, preferablysymmetrically the tire's circumferential center line X--X. Because thegreen tire, with sealant in it, is to be cured in a curing press,essentially no aromatic solvent can be present in the recipe, and noneis added. Only extrusion of the recipe produces a uniformly flat stripof sealant. The liner stock, typically butyl rubber, or carcass stock,typically natural rubber, is subsequently cut into appropriate lengthsof liner 24'. For ease of handling, the liner 24' is placed on apolyethylene backing strip (not shown) which is fed onto a conveyorcarrying the continuous liner stock.

A green carcass is constructed by placing a predetermined length oflaminate 22' on a building drum, preferably superimposing a second innerliner 24 upon the first inner liner, then superimposing the othercarcass components, including belts, bead rings and tread to finish thecarcass. The carcass is then cured conventionally in a curing press sothat the sealant is substantially fully cured to the extent dictated bythe concentration of curing (crosslinking) agent present and theconditions of curing. It will be recognized that when the sealant isfully cured, all the curing agent is used up, but the sealant may notbe, and is not, fully crosslinked.

The uncured sealant has a peak Mooney viscosity at room temperaturegreater than 70 (large rotor, ML), preferably in the range from about 80to about 110. Upon curing, the cured sealant has a peak Mooney viscosityat 150° F. of less than 30, preferably in the range from above about 15but less than 30.

The plastic flow and adhesive qualities of the cured sealant are suchthat it sticks to a puncturing object inserted into it, and, when theobject is withdrawn, the sealant is drawn into the puncture forming aplug which seals the opening against loss of air from within the tire.It is important that the sealant 20 be provided opposite the treadregion 16 of the tire 10 and extend radially inwardly along the buttressregions 17 as well. This location of the sealant layer 20 providesmaximum protection in the puncture region of the tire 10 most vulnerableto puncture by foreign objects. Because the sealant must be maintainedin this location during construction and operation of the vehicle onwhich the tire is mounted, without being covered, and without any edgestrips to restrain flow, the relatively high viscosity of the sealant,before and after it is cured, is of critical importance.

In another embodiment of the invention, the sealant product may beextruded onto a backing strip coated with a detackifier, preferablypoly(vinyl alcohol) ("PVA"), so that the lower surface of the sealantmay be lifted off the backing strip; and, the upper surface of thesealant is likewise coated with an aqueous solution of PVA, and theupper surface is dried in a long convection oven. When a predeterminedlength of sealant is then cut from the continuous extrudate, it iswrapped around the building drum so that the dry detackified surface ofthe sealant is in contact with the drum. The exposed upper surface ofthe sealant is then wiped with a solvent-soaked sponge to free it fromdetackifier. The inner liner is then superimposed upon the sealantstrip, followed by the other tire components.

It is of particular interest that upon curing a green carcass with anintegral sealant/inner liner laminate, the edges of the sealant aresmoothly blended into the sidewalls, but there is so little flow ofsealant under the heat and pressure of curing that the cured thicknessof the sealant is generally not less than 80% of the uncured thicknesswithin a 10 cm band through the center of which runs the mid-equatorialplane X--X.

The necessary viscosity of the sealant, before and after curing, ismaintained by ensuring that there is always more than 50 parts of highmol wt elastomer per 100 parts by wt of combined high and low mol wtelastomers, and preferably twice as much high as low mol wt elastomer inthe recipe; further, by ensuring that there is more processing aid,selected from a homogenizer and a tackifier, than there is high mol wtelastomer. Most preferably, the viscosity of a recipe containing a majoramount of high mol wt elastomer is controlled with the choice of naturalrubber stock and the presence of both, a homogenizer and a tackifier.With a Struktol-MS type homogenizer, it is preferred that there be atleast twice as much homogenizer than tackifier; and, that the combinedwt of homogenizer and tackifier is at least equal to that of the highmol wt elastomer.

The precise ratio of the high to low mol wt elastomers depends mainly onthe mol wt of the high mol wt elastomer and other variables such as theparticular elastomer involved, the amount and kind of crosslinking agent(curative) used, and the conditions for curing the green tire. The ratioof high to low mol wt is preferably chosen so as to give a MLP/rt above70, preferably from about 90 to about 110, for the sealant recipe,before it is delivered to the extruder.

The ratio of high mol wt:low mol wt elastomers is always greater than 1,but may range as high as 10 to 1. A preferred ratio is from greater than1 to about 5, and most preferred is in the range from 2 to about 4.

The MLP/rt viscosity of the sealant recipe, reached at about 1 min orless into the 4 min Mooney curve, most preferably ranges from 95 toabout 105. The viscosity of the extrudate when cooled to roomtemperature is essentially the same as that of the unextruded recipeindicating there is substantially no crosslinking of the sealant afterit is extruded. Though not immediately evident, the viscosity of theextrudate, like that of the mix, is too high to enable it to be extrudedinto a rotating tire, as is taught in the Baboff '172 patent, becausethere is no convenient way of laying down a uniformly distributed stripof such a viscous extrudate, and cutting it off at the desired moment.

After being exposed to tire-curing conditions, typically range fromabout 275° to about 400° F., preferably 350° F., and a pressure in therange from about 200-500 psig, the sealant product in the tire is curedto a MLP/150 of less than 30, preferably from above about 15 to 30.

As the high mol wt elastomer of the sealant product, there may beemployed any high mol wt elastomer capable of being crosslinked.Examples are the highly unsaturated rubbers such as those based onconjugated diolefins, whether homopolymers as in polyisoprene(particularly cispolyisoprene, whether natural or synthetic),polybutadiene (including polybutadiene of high cis content),polychloroprene (neoprene), or copolymers as exemplified by those havinga major proportion of such conjugated dienes as butadiene with a minorproportion of such monoethylenically unsaturated copolymerizablemonomers as styrene or acrylonitrile. Alternatively, elastomers of lowunsaturation may be used, notably butyl type rubbers (copolymers of suchisoolefins as isobutylene with small amounts of conjugated dienes suchas isoprene) or EPDM types (copolymers of at least two differentmonoolefins such as ethylene and propylene with a small amount of anon-conjugated diene such as dicyclopentadiene, 1,4-hexadiene,5-ethylidene-2-norbornene, etc.). Even saturated elastomers such as EPMor ethylene-vinyl acetate may be employed, using the proper cure system.The elastomer may be emulsion-prepared or solution-prepared, stereospecific or otherwise. The mol wt of the solid elastomer is usually inexcess of 50,000 ordinarily within the range of from 60,000 to 2 to 3million or more. Ordinarily the solid elastomeric component has a Mooneyviscosity within the range of from 20 to 160 ML-4 at 212° F.

The low mol wt elastomer employed has a mol wt less than 50,000, usuallywithin the range from 1000 to 10,000, and is preferably of the "liquid"rubber type with a maximum Brookfield viscosity at 150° F. of 2 millioncps, ordinarily within the range of from 20,000 to 1 million cps.Examples are: liquid cis-polyisoprene (e.g. heat depolymerized naturalrubber, or cis-polyisoprene polymerized to low mol wt), liquidpolybutadiene, liquid polybutene, liquid EPDM, and liquid butyl rubber.

The homogenizing agents (homogenizers) used are low mol wt polymericresin blends which assist the high and low mol wt elastomeric componentsto form a homogeneous, uniform, processable mass. The homogenizers areavailable as semi-solid low melting point resins, having varyingpolarities, in block or friable flake form. The homogenizer iscompatible with aliphatic, naphthenic and aromatic portions of theelastomers in a blend. Commercially available homogenizers are Struktol®40MS and Struktol NS, inter alia. Each homogenizer may be used inconjunction with a plasticizer to lower the viscosity or soften theelastomer.

The tackifier is preferably a low mol wt material such as a rosin ester(e.g. Staybelite® Ester 10); aliphatic petroleum hydrocarbon resins(e.g. Piccopale® A-70 and 100S); polyterpene resins derived fromalpha-pinene (e.g. Piccolyte® A-10), beta-pinene (e.g. Piccolyte S-25);resins made from styrene and related monomers (e.g. Piccolastic® A-5);resins made from dicyclopentadiene (e.g. Piccodiene® 2215); and resinsfrom the reaction of a mineral oil purification residue withformaldehyde and with nitric acid catalyst according to U.S. Pat. No.3,544,494 sold under the Struktol trademark).

The sealant recipe includes a sufficient quantity of a curing agent,effective under curing press conditions, so that upon curing, thesealant in the tire is crosslinked to essentially the extent dictated bythe curing agent present. Suitable curing agents are:

1. Sulfur curing systems such as those based on sulfur orsulfur-yielding materials (e.g. tetramethyl thiuram disulfide) andconventional accelerators of sulfur vulcanization.

2. Quinoid curing systems such as p-quinone dioxime (GMF®, from UniroyalChemical) with or without supplementary oxidant.

3. Organic peroxides or hydroperoxides (together referred to as"(hydro)peroxides" for brevity) are free radical generating catalystssuch as dicumyl peroxide, cumene hydroperoxide, methyl ethyl ketonehydroperoxide or other free radical generating catalysts such asazobisisobutyronitrile.

4. Polyisocyanates such as MDI (4,4'-methylene bisphenyleneisocyanate),TDI (toluene diisocyanate), and PAPI (polymethylenepolyphenylisocyanate) as well as dimers and trimers of MDI and TDI.

The amount of curing agent used will vary with the elastomers employedand with their proportions, as well as with the structure of the curingagent, and conditions under which it is used. The amount used issufficient to prevent significant flow of the sealant in a tire attemperatures up to 200° F. and vehicle speeds up to 70 mph while stillretaining sufficient adhesiveness and conformability to perform thedescribed sealant function. The more the high mol wt elastomer,generally the less curing agent required. For a depolymerized naturalrubber (DPR) natural rubber (NR) mixture, the amount of sulfurcontaining or quinoid type curing agent will be in the range of frommore than 0.5 to 2.0 phr (parts per 100 parts by wt of both elastomersadded together), ordinarily from 0.7 to 1.5 phr. For the same mixture,with a polyisocyanate curing agent, the amount required will range from2 to 10 phr, preferably 2.5 to 8 phr. The applicable range for(hydro)peroxide would be 0.1 to 1.0 phr, preferably 0.2 to 0.7 phr, andthe (hydro)peroxides are most preferred. Whichever curative is used, itis essential that the amount used provide the sealant product with acured MLP/150 which is in the range from 15 but below 30.

(Hydro)peroxides include aromatic or aliphatic (hydro)peroxides, forexample the diacyl peroxides, dibasic acid peroxides, ketone peroxides,and alkyl peroxyesters or hydroperoxides; in particular, diacetylperoxide; dibenzoylperoxide; bis-2,4-dichlorobenzoyl peroxide;ditert-buytlperoxide; dicumylperoxide; tertbutylperbenzoate;tert-butylcumylperoxide; 2,5-bis(tertbutylperoxy)-2,5-dimethylhexane;2,5-bis(butylperoxy) dimethylhexyne-3;4,4',4'-tetra(tert-butylperoxy)-2,2-dicyclohexylpropane;1,4-bis(tert-butylperoxy-isopropyl)-benzene;1,1-bis(tert-butylperoxy)3,3,5-trimethylcyclohexane; lauroyl peroxide;succinic acid peroxide; cyclohexanone peroxide; tert-butylperacetate;butyl hydroperoxide; and the like.

The amount of (hydro)peroxide used is preferably from 0.8 to 3.0millimoles (mmol), more preferably from 0.9-2.6 mmol per 100 g of highmol wt elastomer. Specifically with dicumyl peroxide, from 1.4-2.4mmol/100 g maintains a MLP/150 in the range from 15 to less than 30,preferably 17-28, most preferably 18-25.

It is desirable to add an antioxidant, antiozonant or other stabilizerin the sealant recipe, but it is not desirable to add a pigment such ascarbon black, or any particulate inorganic fillers or extenders, all ofwhich tend to denigrate the efficacy of the sealant; neither is itdesirable nor necessary to add fibrous fillers to the sealantcomposition because these adversely affect the control of viscosity inthe necessary range.

In practicing the invention, the ingredients are mixed togetheruniformly in a sigma-bladed Baker-Perkins blender, internal Banburymixer, or the like, and the resulting mixture (recipe) is extruded toform a flat strip about 0.1 inch to about 0.5 inch thick on thelaminate.

The puncture sealant ability and resistance to flow of the sealant istested in an inflated tire run at a rotational speed equivalent to avehicle speed of 50 to 75 mph and a load sufficient to generate aninternal temperature of 200° F. After the run, the tire is examined tosee if the sealant has flowed out of the shoulders and into the crownarea, or whether the sealant has formed a puddle in the bottom of thetire. Puncture sealant ability is evaluated by puncturing the tire withnails of different sizes which are subsequently removed from the tire,and the loss of air pressure within the tire is measured.

The following examples will serve to illustrate the practice of theinvention in greater detail.

EXAMPLE 1-6

A sealant containing 62 parts of natural rubber (NR), 43 partsdepolymerized rubber DPR-400, 64 parts Struktol 40MS homogenizer, and 21parts Piccopale 100SF tackifier, was blended with 0.5 parts stabilizer;then, 0.5 parts dicumylperoxide in a 40% concentration were added. TheNR is standard Malaysian with ML-4(100° C.) of about 60. The viscosityof the DPR-400 is 80,000 cp at 150° F. (66° C.). The stabilizer is amixture of 40% distearylthio dipropionate, 20% blend of substitutedphenylenediamine antiozonants, 20% nonylated phenylphosphiteantioxidant, and 20%thiodiethylenebis(3,5-di-t-butyl-4-hydrophenylpropionate) antioxidant.

The mix was extruded as a layer about 9" (ins.) wide and 0.35" thickonto a continuous web of inner liner 11 ins wide, to form a laminate,the upper surface of which is sprayed with an aqueous solution of PVAand dried. The laminate was then used to build a carcass of aself-sealing 225HR60-15 tire. The MLP/rt of the mix was 100, and that ofthe extrudate was essentially the same. After the tire is expanded inthe second stage, the thickness of the sealant is about 0.2" exceptwithin about 0.5" of the edges where they are tapered and blended intothe sidewalls. The cured sealant thickness is found to be about 0.2"with the edges of the sealant remaining smoothly faired into thesidewalls. A portion of the sealant, when cured at 182° C. for 15minutes, has a peak MLP/150 of 28.

The tire was mounted on a Getty wheel and inflated. As a measure ofsealing efficiency, sixteen 20d nails, shank diameter of 0.185" and 2.5"long, were driven into the tire, some in the outer ribs, some in theouter grooves and others in inner positions through lugs and grooves.The tires were spun at various speeds corresponding to vehicle speedsranging from 50 to 75 mph (ca 80-120 km/hr) causing all the nails to beexpelled by centrifugal force. The number of sealed punctures inrelation to the total number of punctures (16) was determined, and theresults were listed as a ratio of seals/punctures.

Though the ratio of NR to DPR is the same in all tests (62/38), and theratio of homogenizer to tackifier is not varied in the 6 examples ofsealant product tested, the sensitivity of the sealant to the amount ofdicumyl peroxide ("dicup") under identical curing press conditions, isillustrated in the following Table I. A rating of 8 successfully sealedpunctures out of 16 (50%) is deemed unsatisfactory, 9 out of 16 isacceptable, and 10 or more out of 16 is preferred. Most preferred is atleast 13 out of 16.

                  TABLE I                                                         ______________________________________                                               Run No.                                                                       1     2       3       4     5     6                                    ______________________________________                                        Dicup, parts                                                                           0.46    0.61    0.78  0.93  1.07  1.39                               MLP/150  16      18      20    23    25    30                                 Seals/   12/16   14/16   15/16 14/16 14/16 8/16                               punctures                                                                     ______________________________________                                    

Examples 7-12

Additional self-sealing tires were constructed using a sealant in whichthe ratio of high mol wt (NR)/low mol wt (DPR), and the ratio ofhomogenizer(Struktol MS)/tackifier (Piccopale A-70), were varied. Theamount of stabilizer (same as used hereinabove) and dicup were keptsubstantially constant. The recipes used, and the results obtained areset forth hereinbelow in Table II.

                  TABLE II                                                        ______________________________________                                               Run No.                                                                       7     8       9       10    11    12                                   ______________________________________                                        High mol wt                                                                            57      61      64    68    83.4  90                                 (NR)                                                                          Low mol wt                                                                             43      39      36    32    16.6  10                                 (DPR-400)                                                                     Struktol 37      65.1    58    58    53    63                                 MS-40                                                                         Piccopale                                                                              20      32.6    28    28    32    42                                 100S                                                                          Stabilizer                                                                             0.6     0.6     0.6   0.6   0.6   0.6                                Dicup, parts                                                                           1.0     1.0     1.0   1.0   1.0   1.0                                MLP/150  20      18      20    19    26    28                                 Seals/   14/16   12/16   14/16 14/16 14/16 14/16                              punctures                                                                     ______________________________________                                    

What is claimed is:
 1. A sealant product consisting essentially ofanextruded, generally flat strip of a surface-detackified homogeneousmixture of (i) high molecular weight and low molecular weight elastomerspresent in a weight ratio (high mol wt/low mol wt) greater than 1; (ii)a processing aid selected from a homogenizer and a tackifier, providedthat the combined weight of homogenizer and tackifier is at least equalto that of the high molecular wt elastomer; (iii) a curing agent,effective at a temperature in the range from about 250°-450° F. and apressure in the range from about 200-500 psig to substantially fullycure said high molecular wt and low molecular wt elastomers to theextent determined by the molar amount of curing agent which is presentin the range from about 0.8 to about 3.0 millimoles per 100 g of highmolecular wt elastomer; and, (iv) a stabilizer in an amount from about0.1 to about 1.0 part per 100 parts of combined high and low molecularwt elastomers; said mixture having a room temperature peak Mooneyviscosity in the range from above 70 to about 110, which viscosity issubstantially the same after said mixture is extruded; and, said striphaving its upper and lower surfaces coated with a detackifier in anamount effective to negate adhesion of the surfaces to metal, saiddetackifier being removably coated upon one surface of said strip. 2.The sealant product of claim 1 wherein said high molecular weightelastomer has a Mooney viscosity of from 20 to 160 ML-4 at 212° F., thelow molecular weight elastomer is a liquid rubber having a Brookfieldviscosity at 150° F. of from 20,000 to 2,000,000 cps, and the curingagent is selected from the following, present in the amountsrecited:from more than 0.5 to 2.0 parts of sulfur or sulfur-yieldingcurative; from more than 0.5 to 2.0 parts of quinoid curative; from 0.1to 1.0 part of a free radical generating curative; and, from 2 to 10parts of polyisocyanate curative.
 3. The sealant product of claim 2wherein the liquid rubber is heat depolymerized natural rubber.
 4. Thesealant product of claim 2 wherein the low molecular weight elastomer isselected from the group consisting of liquid cis-isoprene, liquidpolybutadiene, liquid polybutene, liquidethylene-propylene-non-conjugated diene terpolymer rubber, and liquidisobutylene-isoprene copolymer rubber.
 5. The sealant product of claim 2wherein the high molecular weight elastomer is selected from the groupconsisting of conjugated diolefin homopolymer rubbers, copolymers of amajor proportion of a conjugated diolefin with a minor proportion of acopolymerizable monoethylenically unsaturated monomer, copolymers ofisobutylene with a small amount of isoprene,ethylene-propylene-non-conjugated diene terpolymers, and saturatedelastomers.
 6. The sealant product of claim 2 wherein said homogenizerand said tackifier are each present and there is more homogenizer thantackifier, by weight, present.
 7. The sealant product of claim 2 whereinsaid homogenizer is a low molecular weight polymeric resin blend adaptedto coalesce and mix the high and low molecular weight elastomers andtackifier into a uniform mass.
 8. The sealant product of claim 2 whereinsaid tackifier is selected from the group consisting of a rosin ester,aliphatic petroleum hydrocarbon resin, polyterpene resin derived fromalpha-pinene, beta-pinene, resins made from styrene and relatedmonomers, resins made from dicyclopentadiene, and, resins from thereaction of a mineral oil purification residue with formaldehyde andwith nitric acid catalyst.
 9. The sealant product of claim 2 whereinsaid free radical generating catalyst is an aromatic peroxide.
 10. Thesealant product of claim 2 wherein said detackifier is poly(vinylalcohol).
 11. An elastomeric laminate comprising,(a) an air-impermeableelastomer strip for use in an article to be cured under heat andpressure; (b) a layer of puncture sealant adhered to the upper surfaceof said elastomer strip, said sealant covering an area from about 20% tosubstantially the entire area of the strip, said sealant consistingessentially of a homogeneous mixture of (i) high molecular weight andlow molecular weight elastomers present in a weight ratio (high molwt/low mol wt) greater than 1; (ii) a processing aid selected from ahomogenizer and a tackifier, provided that the combined weight ofhomogenizer and tackifier is at least equal to that of the highmolecular wt elastomer; (iii) a curing agent, effective at a temperaturein the range from about 250°-450° F. and a pressure in the range fromabout 200-500 psig to substantially fully cure said high molecular wtand low molecular wt elastomers to the extent determined by the molaramount of curing agent which is present in the range from about 0.8 toabout 3.0 millimoles per 100 g of high molecular wt elastomer; and, (iv)a stabilizer in an amount from about 0.1 to about 1.0 part per 100 partsof combined high and low molecular wt elastomers; said mixture having aroom temperature peak Mooney viscosity in the range from above about 70to about 110, which viscosity is substantially the same after saidmixture is extruded; and, (v) said layer of sealant having its uppersurface coated with a detackifier in an amount effective to negateadhesion of the detackified surface to metal.
 12. The sealant product ofclaim 11 wherein said high molecular weight elastomer has a Mooneyviscosity of from 20 to 160 ML-4 at 212° F., the low molecular weightelastomer is a liquid rubber having a Brookfield viscosity at 150° F. offrom 20,000 to 2,000,000 cps, and the curing agent is selected from thefollowing, present in the amounts recited:from more than 0.5 to 2.0parts of sulfur or sulfur-yielding curative; from more than 0.5 to 2.0parts of quinoid curative; from 0.1 to 1.0 part of a free radicalgenerating curative; and, from 2 to 10 parts of polyisocyanate curative.13. The sealant product of claim 12 wherein the liquid rubber is heatdepolymerized natural rubber.
 14. The sealant product of claim 12wherein the low molecular weight elastomer is selected from the groupconsisting of liquid cis-isoprene, liquid polybutadiene, liquidpolybutene, liquid ethylene-propylene-non-conjugated diene terpolymerrubber, and liquid isobutylene-isoprene copolymer rubber.
 15. Thesealant product of claim 12 wherein the high molecular weight elastomeris selected from the group consisting of conjugated diolefin homopolymerrubbers, copolymers of a major proportion of a conjugated diolefin witha minor proportion of a copolymerizable monoethylenically unsaturatedmonomer, copolymers of isobutylene with a small amount of isoprene,ethylene-propylene-non-conjugated diene terpolymers, and saturatedelastomers.
 16. The sealant product of claim 12 wherein said homogenizerand said tackifier are each present and there is more homogenizer thantackifier, by weight, present.
 17. The sealant product of claim 12wherein said homogenizer is a low molecular weight polymeric resin blendadapted to coalesce and mix the high and low molecular weight elastomersand tackifier into a uniform mass.
 18. The sealant product of claim 12wherein said tackifier is selected from the group consisting of a rosinester, aliphatic petroleum hydrocarbon resin, polyterpene resin derivedfrom alpha-pinene, beta-pinene, resins made from styrene and relatedmonomers, resins made from dicyclopentadiene, and, resins from thereaction of a mineral oil purification residue with formaldehyde andwith nitric acid catalyst.
 19. The sealant product of claim 12 whereinsaid free radical generating catalyst is an aromatic peroxide.
 20. Thesealant product of claim 12 wherein said detackifier is poly(vinylalcohol).